Is 18mm plywood enough for a cold water tank support base? And what type of ply?

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Krome10

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Hi again

Another bathroom refit question, this one involving wood but still off topic enough for this sub forum I think :)

With the upcoming plumbing work and everything being drained down for it, I'm taking the opportunity to replace the boards under the tank in the loft because they have signs of woodworm... I'm hoping to use 18mm ply because it is available at B&Q where they will cut it to size for free, which will save time, hassle, and will mean I can fit it in the car! But is 18mm thick enough?

Here's some relevant info:

- The existing base is T&G floorboards and they're 20mm
- The tank spans three joists
- The joists are 150mm x 37mm. Joist centres are average 400mm apart.
- There are no walls beneath the tank. The nearest wall is a solid internal wall, which is 500mm away from the tank, to the side.
- The tank is 40 gallon / 182L actual (60 / 273 nominal)

If 18mm will be enough, it seems B&Q offer two types of ply and I wondered which is best please?



Many thanks for the help
 

Fitzroy

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The load will be on the joists not the board, so board thickness is irrelevant from a tank perspective. My old water tank sat on the joists in an unboarded loft.
 

Jones

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Board spreads the load onto the joists to prevent point loading on the tank. Chip board will be cheaper than ply and the diy sheds sell it in small boards that are easy to get through the loft hatch. For a 400mm span 18 mm is ok
 

shed9

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Those joists are fairly small in dimensions and will give a max span of <3m. You need to know the span as well as the distance between individual joists. That's about 200kg of dead weight and it's position along that span is also important, relevant to the tanks footprint real estate. As above the board is just spreading the weight so 18mm is ample.
 

Sandyn

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Having lived in the same house for 40+ years, I have seen the results of things I did years ago, which could have been prevented by doing something very simple at the time. When I do anything now, I think 40 years ahead, so I don't pass on problems to future owners.
Remember the risk of water leakage/ condensation drips from the tank at some point in the future. You could put some polythene below the tank to protect whatever material you use. I have seen chipboard crumble after years of slight dampness. I would use 18mm T&G flooring spanning 3 joists and well screwed down.
(If I was doing this just now, I would go on to facebook and get a couple of free pallets. and use those. recycle/reuse and free)
 

Krome10

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Many thanks for the replies

One of the advantages of the ply is that B&Q can cut it to the required length which means it will fit in the car (and save me time and hassle cutting it myself). I also think there are advantages from a weight bearing perspective in having the base as one single piece.

I'm steering away from chipboard as I'd worry that a small persistent leak could be soaked up by it and become a problem well before I'd notice anything in the room below.

Out of the two available at B&Q which would you say is the better in my situation? I was originally thinking to treat the ply to make it waterproof, but wondering now if that's overkill. Especially base don the chipboard suggestions....


You need to know the span as well as the distance between individual joists.... position along that span is also important, relevant to the tanks footprint real estate.

The tank footprint is 900 x 500mm. To one side there is an internal solid wall, which is 500mm away from the side of the tank. Seems the joists rest directly on that wall. But then resting on the joists is a large purlin which is supporting the roof.

It's the same on the other side, and I'll add a photo of that. The purlin on the other side is 1000mm away from the side of the tank. However, the wall (external) on that side is lower, as the ceiling slopes down to it. So the joists don't sit on the wall on that side, but join the rafters on the far side of the chunky purlin. I think I've got my terms right! But let me know if none of that makes sense!

Here's those photos:

IMG_20211118_104034382.jpg

IMG_20211118_104351948.jpg


Thank you all for the help :)
 

Stevekane

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Im sure this is something you've covered but just as a point of interest, it seems unuseual to have water tanks anymore, everyone seems to have gone over to combi boilers, its not now just a coldwater storage tank that you could get rid of?
Steve.
 

Jones

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I'd add a bit of insulation while you're up there. It looks like you've got about 3" of loose fill in between the rafters and an inch or two on the felt. Rolling out 8" of rockwool will make a lot of difference and if you're lucky someone else will pay for it.
 

Krome10

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Im sure this is something you've covered but just as a point of interest, it seems unuseual to have water tanks anymore, everyone seems to have gone over to combi boilers, its not now just a coldwater storage tank that you could get rid of?
Steve.

Thanks for the heads up, but definitely in use as we're on a vented system (gravity).

I'd add a bit of insulation while you're up there. It looks like you've got about 3" of loose fill in between the rafters and an inch or two on the felt. Rolling out 8" of rockwool will make a lot of difference and if you're lucky someone else will pay for it.

It's on the (ever growing) To Do List! As is lagging the pipes, and getting better insulation for the tanks. For the matter a re-roof too! Thank you for pointing it out though.
 

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